As pointed out in my copending application Ser. No. 661,307, filed Feb. 25, 1976, and the application of Virgil L. Frantz on a Dual Filter Assembly, Ser. No. 766,437, filed Feb. 7, 1977, a particulate desiccant, while highly efficient in selectively adsorbing from compressed gas an entrained component, such as moisture, due to the rubbing together of the desiccant particles, tends to release dust into the gas being filtered, usually to the detriment of devices activated by the filtered gas. This tendency is minimized by including in the filter unit a pneumatic compactor such as disclosed in my application Ser. No. 661,307. However, if, as illustrated in that application, the compactor acts through an interposed floating follower on the desiccant, in a direction opposite that in which the canister is inserted in the housing, given a high pressure, high flow installation such as in the above Virgil L. Frantz application, the effect on the canister containing the desiccant can be to displace the canister from its seat in the housing sufficiently to permit a substantial portion of the compressed gas introduced for filtering into the housing of the filter unit, to bypass the desiccant and be discharged without filtering from the housing. It is with preventing this phenomenon, that the compactor of the present application is particularly concerned.